Having driven in a vehicle with both, here goes:
* a locker will allow you to go further in 2wd than 4wd with a limited slip...and get into more trouble
* 4wd is not really 4wd...it's 2wd (1 front and 1 rear).
* limited slip is better in snow/ice than a locker (paved road snow/ice...offroad snow/ice = a locker is much better).
* a locker will require a cable (OX) or electrical switch (ARB).
Without knowing the specifics of the van and the driving type or location, I'd choose locker.
What axles are under the van?